Polarizing beam splitter assembly with diffracting element

ABSTRACT

A polarizing beam splitter assembly for directing image light on an input path into multiple exit light paths comprises multiple prisms with edges that meet to form a seam. The polarizing beam splitter assembly includes a diffracting element prior to the seam in the input light path. The diffracting element comprises a geometry that performs at least one of blocking a portion of the image light and scattering a portion of the image light.

TECHNICAL FIELD

This disclosure relates to projection systems for projecting images fora three-dimensional viewing experience, and more particularly relates tosuppressing image defects generated by a polarization beam splitter.

BACKGROUND

In recent years, cinemas and image projection systems have used 3Dprojection systems that use polarized light to encode the left-eye andright-eye images to form stereoscopic imagery when viewed throughpolarization filtering eyewear. In some cases, light-doublingpolarization projection systems such as those available from RealD Inc.have used multiple paths to minimize light intensity losses frompolarization control components, thereby maximizing the amount of imagelight that reaches the screen to create a brighter cinematic experiencefor the viewer. Some examples of light-doubling systems include thosetaught by commonly-owned U.S. Pat. No. 7,905,602, commonly-owned U.S.Pat. No. 8,220,934, commonly-owned U.S. Pat. No. 7,857,455, andcommonly-owned U.S. Pat. No. 9,958,697, all of which are hereinincorporated by reference in their entireties.

Such light-doubling systems utilize a Polarizing Beam Splitter (PBS)that generally splits input image light into multiple directions. Insome systems, e.g., two-beam systems, input image light is split intotwo directions—a transmitted path and a reflected path, each path havinga different polarization state. In other systems, known in the industryas three-beam systems, there may be a transmitted path and two reflectedpaths. In such systems, the light in each path may be processed bypolarization control components and then the polarization-encoded imagelight is recombined on a polarization-preserving screen to be viewed bya person with polarization filtering eyewear.

SUMMARY

Disclosed herein is a polarizing beam splitter (PBS) assembly fordirecting image light on an input light path into multiple exit lightpaths. The PBS assembly may include a top prism having a first surfaceadapted to receive the image light on the input light path. The topprism may also include a second surface adapted to direct the imagelight along a first exit light path. The first and second surfaces ofthe top prism may meet to form a first edge.

The PBS assembly may also include a bottom prism having a first surfaceadapted to receive the image light on the input light path. The bottomprism may also include a second surface adapted to direct the imagelight along a second exit light path that is different from the firstexit light path. The first and second surfaces of the bottom prism maymeet to form a second edge. The first edge, which is on the top prism,and the second edge, which is on the bottom prism, may meet to form aseam.

The PBS assembly may also include a center prism between the top prismand the bottom prism. The center prism may have a first surface adjacentto the second surface of the top prism, and the center prism may have asecond surface adjacent to the second surface of the bottom prism. Thefirst and second surfaces of the center prism may meet to form a thirdedge. The third edge, which is on the center prism, may meet the firstedge, which is on the top prism, and the second edge, which is on thebottom prism, at the seam.

The PBS assembly may also include a diffracting element aligned with theseam prior to the seam in the input light path. The diffracting elementmay comprise a geometry that performs at least one of blocking a portionof the image light and scattering a portion of the image light.

The PBS assembly may also include a planarization plate prior to theseam in the input light path. The planarization plate may be adjacent tothe first surface of the top prism and to the first surface of thebottom prism. The planarization plate may be made of optical gradeglass, such as Schott BK7. In some embodiments, the diffracting elementis located on a surface of the planarization plate. In some embodiments,the diffracting element comprises a chrome or chrome oxide featureproduced by a photolithography process.

Also disclosed herein is a stereoscopic image apparatus which includes aPBS assembly as described herein. The stereoscopic image apparatus mayalso include one or more reflective members in one or more of the exitlight paths. Light reflected by the first and second reflective membersmay form a single stereoscopic image on an image-forming surface, suchas a screen. In some embodiments, the stereoscopic image apparatus mayalso include one or more polarization modulators adapted to switch thepolarization state of the image light on at least one of the first exitlight path and the second exit light path between a first state ofpolarization and a second state of polarization.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example synchronized, high resolution 3Dprojection system utilizing 3D hardware including a PBS for separatinglight into multiple paths;

FIG. 2A illustrates a cross-sectional view of an example PBS derivedfrom three individual prisms;

FIG. 2B illustrates a first of three individual prisms in an examplePBS;

FIG. 2C illustrates a second of three individual prisms in an examplePBS;

FIG. 2D illustrates a third of three individual prisms in an examplePBS;

FIG. 3 illustrates an example planarization plate;

FIG. 4A illustrates a cross-sectional view of an example planarizationplate attached to three prisms of an example PBS;

FIG. 4B illustrates the location of a center seam of an example PBSassembly derived from three individual prisms and a planarization platewhen bonded together;

FIG. 4C illustrates a magnified view of a center seam;

FIG. 5 illustrates an example PBS assembly with a magnified view of atypical observation of a PBS seam;

FIG. 6 illustrates an example synchronized high resolution 3D projectionsystem utilizing 3D hardware and demonstrating an example of on-screenbanding caused from a PBS seam;

FIG. 7 illustrates an example of image defects as observed on a screenby a viewer;

FIG. 8 illustrates examples of geometries that, when aligned to the PBSseam, may suppress effects caused by the seam;

FIG. 9 illustrates example diffraction behavior;

FIG. 10 illustrates an example of image defect suppression when ageometry has been aligned to a PBS seam;

FIG. 11 illustrates a sinusoidal geometry patterned over a PBS seam;

FIG. 12 illustrates an example of on-screen “Image Flare” suppression;

FIG. 13 illustrates a projection system test setup that may predict theobserved on-screen effect of a PBS seam when the PBS is present withinthe test setup light path; and

FIG. 14 illustrates variables that may be important to consider in thedesign of a geometry for use in suppressing on-screen effects caused bya PBS seam.

DETAILED DESCRIPTION

Disclosed herein is a PBS assembly utilizing an optical bonding processto improve and overcome potential deficiencies in the PBS assembly thatmay result in on-screen image smearing and banding. The image smearingand banding may be a direct result of, for example, image splitting andindex mismatching failures that may occur at the optical junction ofthree individual prism alignments. A theoretically perfect PBS assemblywould have no space between the two prisms defined as the top and bottomprisms in the assembly. The top and bottom prism edges may mate to thecenter prism edge at the input side of the PBS. A space or gap at thislocation may result in a materials index mismatch due to a PBS coatingthat may be present on the prisms. The PBS coating may be, for example,a multi-layer thin film dielectric stack, a wire grid thin film metal,or any other appropriate coating. An index mismatch can also be causedby other assembly defects such as trapped gas or foreign contaminants,for example.

The PBS design may utilize a PBS coating, such as a multilayer thin filmreflective coating or wire grid thin film metal, to separate the singleinput light path into multiple paths. These special coatings may beapplied to some of the prism surfaces that comprise the PBS. If fullcoating coverage does not occur or is non-uniform in coating thickness,an index mismatch may result in some regions of the prism surfaces.Coating deposition errors for the individual prisms that comprise thePBS and/or errors in the manufacturing assembly processes for the PBSmay result in index mismatching at the prism junctions which may, inturn, result in light diffraction and/or refraction at that location.

Suppression of the light diffraction, which may be caused, for example,by splitting the input image light and/or by an index mismatch in thePBS assembly, may be achieved by blocking and/or scattering the imagelight prior to the central PBS prism junction, thus controlling thediffraction behavior of the light. The bonding technique may be achievedby first creating a narrow light blocking and or scattering geometry atthe image light input side of the PBS.

One type of PBS may be fabricated by bonding optical prisms together toform a single bonded optic. One function of a PBS is to split light intotwo or more optical paths. Some theater projection systems utilizehardware that converts outgoing light images to right and left handcircular polarization (CP) states to be received by a viewer wearing CPeyewear that decodes the left and right polarization states. Suchtheater projection systems thus yield stereoscopic three dimensional(3D) images. In many of these systems, light loss exists due to, forexample, linear polarization films in the 3D hardware and/or losses fromother materials and optical components within the light paths.

Multiple path systems that overlay images from separate paths increasebrightness and system light efficiency, and are sometimes referred to aslight recycling systems. The splitting of light is achieved by using aPBS that separates input image light into multiple exit paths.Recombining paths at the screen increases the total light efficiency andreduces light loss caused by linear polarizers and/or additional opticalcomponents.

Three-path optical systems that use a PBS assembled from three separateprisms may suffer from a number of deficiencies. In some applications,one or more of these deficiencies manifest as a visible on-screen defectobservable by a viewer of content transmitted and/or reflected through aPBS. Input image light may be completely passed through one path of thePBS (the transmitted path). Also, the input image light may be separatedby angled surfaces on which PBS coatings may have been deposited,therefore creating two reflected paths. One fundamental deficiency thatmay be associated with this approach to light beam separation is thatdiffraction caused by the edge boundary may be introduced when splittinga single image into multiple images. At the junction of the threeprisms, there may exist a region of refractive index (RI) mismatch. Thisregion may create a visible seam, introducing additional sharp edgeboundaries which may cause diffraction. In this disclosure, the term“seam” is used to define the junction of the prisms, which may includesuch a region of potential RI mismatch. The light diffraction at the PBSseam may result in a vertical smearing of images on-screen that can bereadily observable by a viewer under some viewing conditions. Thediffraction may also cause a visible horizontal banding across the fullcinema screen that can appear, for example, as a dark grey band or acolored band depending on the input light source. These effects may bereceived as a visual hindrance to most 3D viewers and may require asolution to vastly suppress or eliminate the on-screen visibility.

Disclosed herein is a solution to largely suppress the unidirectionalimage smearing and banding generated at the seam of a PBS. The solutiondisclosed may be implemented in the assembly process and may suppressthe inherent diffraction caused by splitting the input image light.Further, diffraction may be caused or worsened by poor index matching atthe prisms seam due to, for example, abrupt changes in the PBS coatingthickness, incomplete PBS coating coverage, and/or too large of a spacebetween the top and bottom prisms within the PBS assembly.

FIG. 1 illustrates an example sequential high resolution 3D system 100utilizing a digital theater projector 110, polarization controlledstereoscopic 3D hardware unit 120 operating at 144 Hz, and a screen 130where 3D content is viewed and located downstream of the projectionsystem. In this example illustration, the 3D hardware is a three-pathsystem that separates image input light 150 into right-eye and left-eyepolarization states that are delivered to the screen 130, reflected, andthen decoded with the viewer's cinema eyewear 160. If the 3D hardwareunit 120 does not implement the PBS assembly utilizing an opticalbonding process to improve and overcome potential deficiencies in thePBS assembly disclosed herein, image defects may be observed at thescreen 130 by the viewer 140.

In addition to a PBS assembly, in some embodiments the hardware unit 120includes at least one of (1) at least one reflective member, for examplea mirror, to modify light reflected by the PBS to form a singlestereoscopic image on an image-forming surface such as the screen 130,(2) at least one polarization modulator to modulate the light reflectedby the reflective member and the light transmitted through the PBS, and(3) at least one refractive member to refract light.

FIG. 2A illustrates a PBS assembly 200 incorporating three individualprism components. FIG. 2B illustrates a top prism 225 defined by thegiven angle set (A, B, and C) and leg lengths (a, b, and c) for thisprism. A PBS coating, such as a multi-layer thin film reflective coatingor wire grid thin film metal, may be deposited on the surface defined byleg (a). Leg (a) of prism 225 may be the surface that is opticallybonded to leg (b) of prism 250 using an index-matched optically clearadhesive. FIG. 2C illustrates a center prism 250 defined by the givenangle set (A, B, and C) and leg lengths (a, b, and c) for this prism.FIG. 2D illustrates a bottom prism 275 defined by the given angle set(A, B, and C) and leg lengths (a, b, and c) that may be equal to that ofprism 225. A PBS coating may be deposited on the surface defined by leg(a). Leg (a) of prism 275 may be the surface that is optically bonded toleg (c) of prism 250 using an index-matched optically clear adhesive.This PBS design can be used in stereoscopic 3D hardware image and videogeneration and incorporated in cinema theaters world-wide.

FIG. 3 shows an example planarization plate 300 comprising glass havinga height 320, a length 330, and a thickness 340. The glass can be anyoptical grade glass; however, Schott BK7 or equivalent may be preferreddue to its exceptionally high transmission and ability to acceptprecision polishing for yielding a very high level of surface flatness.A multi-layer thin film low reflectance anti-reflection (AR) coating 310may be deposited on at least one side of the planarization plate tominimize the reflection at the interface of the projection lens.

FIG. 4A shows a cross-sectional view of an example PBS assembly 400incorporating three individual prism components 225, 250, 275 and aplanarization plate 300. The planarization plate 300 may be centeredabout the input surface. The dimensions of the planarization plate 300can change for system design and assembly considerations. FIG. 4Billustrates a cross-sectional view of the location of a seam 410 of thethree edges of a bonded prism set at the interface of the entranceplanarization plate 300 and prism set 225, 250, 275. This is a locationwhere light diffraction may occur, causing unsightly image defectson-screen that can be observed by a viewer. FIG. 4C depicts across-sectional magnified view of the location of the seam 410 of thePBS assembly 400.

FIG. 5 illustrates the seam of a fully assembled example PBS 500 thatalso includes a magnified view 510 of the seam. In this example, themagnified view 510 depicts a light grey region 520 bordered by dark greyregions 530 where the sharp boundaries define the narrow region of RImismatch. This narrow region can vary in width across the PBS 500 whichcan directly result in increased or decreased levels of scatter observedon-screen.

Image splitting may cause light diffraction at the seam. RI mismatch atthe seam may result in additional light diffraction about the mismatchboundaries. The resulting on-screen image defect may be referred to as“Image Flare” or scatter. “Image Flare” may be defined as a verticalghost image that diffusely projects, or smears, above and below thecenter of the image. The visibility of the “Image Flare” may be directlyrelated to the size (width) of the RI mismatch region. For example, ifthe RI mismatch region running parallel to the PBS input seam is narrow,the observed “Image Flare” on-screen may be minimized. Likewise if theRI mismatch region running parallel to the PBS input seam is wide, theobserved “Image Flare” on-screen may be maximized. The visibility of the“Image Flare” can be dependent on other factors. For example, in a highdynamic range (HDR) theater setup the “Image Flare” can become morevisible. Another example of how “Image Flare” may be more easilyobserved is when using a high contrast projection lens. “Image Flare”may also be more prominent with high contrast content. An example ofhigh contrast content is white text on a black background.

FIG. 6 illustrates an example 3D projection system 600 that includesthree-beam 3D polarization conversion hardware 620 where the on-screen630 diffractive banding effect 660 may be caused by the diffraction fromthe PBS seam. The PBS may be located within the 3D hardware unit 620.The severity of the banding 660 may be affected, for example, by the PBSseam characteristics, 3D hardware 620 positioning, projector 610 type,projector lens, projector light source, projector light brightness,throw ratio, projector content 650, theater conditions and othervariables. When projecting high contrast images the defect visibilitymay be far more severe. Likewise, the source type can add visualalterations to the observed banding 660. For example, a Xenon lamp mayresult in on-screen banding that appears as dark grey to light greytransitions, whereas a red, green, blue laser source may result inon-screen chromatic banding transitions. The resulting on-screen bandingcaused by the light diffraction at the seam can appear visuallydifferent due to many contributing variables.

FIG. 7 illustrates an example of “Image Flare” as it appears on-screen700 when viewing normal to the screen. In this example, high contrastcontent that consists of white squares on a black background is beingdisplayed in an HDR setup. Within the projection system setup, the 3Dhardware may be positioned so that the white boxes are centeredhorizontally to the center of the seam of the PBS. The severity of the“Image Flare” may be dependent on contributions of the separate pathsand other contributing factors previously discussed. FIG. 7 demonstratesdiffering severity of “Image Flare” in both top and bottom locations offthe left, center, and right white squares observed. The “Image Flare”observed in FIG. 7 may be caused directly by light diffraction at theinput seam interface of the PBS.

Suppressing diffraction generated at the seam may require properassembly of the three prisms that comprise the PBS as well as a thinline geometry diffracting element aligned with the seam. Thisdiffracting element may be added onto the planarization plate, wherethis thin line geometry diffracting element can be well aligned to thePBS seam. This geometry is designed to mask the visibility of the “ImageFlare” resulting from both the PBS seam and the inherent diffractioncaused by the upper and lower path light splitting.

With respect to the assembly of the three prisms, firstly the prismcoating should have complete coverage of the prism surface. Thin filmcoating processes may require the glass surface to be held or fixturedin the coating chamber and may require special masks that control thethin film coating placement. Coating edge effects can occur as well; forexample, chips and scalloped edges may impact thin film coating byresulting in coating non-uniformities at such edge effect locations.These are examples of coating process details that could result inincomplete or inadequate coating coverage on a glass surface. A fullcoverage coating may be required in order to ensure proper indexmatching within the assembly.

Secondly, it is also important to maintain a precision edge on theprisms. Imperfections on the edges that form the seam can be caused bypolishing errors, handling damage, pyramidal defects, and rounded edges.

If prisms that have full coating coverage and precision edges are chosento be used in the assembly of a PBS, they should be then mated togetherin a precise manner using a thin layer of optical bonding adhesive.During the assembly, no space should exist between the coated prismedges. Critical fixturing may be necessary to maintain and hold properalignment of the individual prisms during the assembly and adhesivecure.

The approach to suppress diffraction disclosed herein may utilizewell-controlled and dimensioned geometries placed at the central inputof the PBS, and aligned to the PBS seam. It should be obvious to thosefamiliar with the relevant art that there are multiple surfaces on whichthe geometries can reside. Examples of possible surfaces where thegeometries could be located are the entrance surface of the PBS, thebonded surface of the planarization plate, and the entrance surface ofthe planarization plate. This approach may offer a solution that can beeasily implemented into an existing PBS assembly process. Preferably,the geometry is located in close proximity to the seam.

Diffraction is the spreading of light waves caused by obstacles, edges,and openings in a light path. Because light propagates as a wave, itspreads after passing an edge. The edge boundary characteristics maydetermine the directionality of the light scatter. In the case of alinear sharply defined edge in a horizontal orientation (as is the casewith the PBS), the light waves will scatter in the vertical direction.The human eye is quite effective at detecting straight lines and linearfeatures. The unidirectional nature of the scatter generated at the PBSseam is particularly noticeable on-screen by viewers. The solutiondisclosed herein deliberately introduces a diffracting element thatredirects scatter in random directions. There are considerations in thedesign of the geometry that may suppress the diffraction. These includebut are not limited to the geometry material, geometry coatingthickness, geometry size, edge feature frequency, edge feature phase,edge profile, edge feature frequency and amplitude variation, and thegeometry width-to-edge profile ratio. The material design can be opaque,semi-opaque, or transparent and can have a smooth or rough surface andincorporate diffuser type elements.

The edge geometry is a primary variable in controlling scatter. A goalof the edge geometry is to uniformly scatter light in many directions tosuppress the directionality caused by a sharp linear edge boundary.Examples that could be considered as an edge shape include, but are notlimited to, a sawtooth, offset sawtooth, sine wave, offset sine wave,square wave, offset square wave, stepped square wave, individual linesat differing orientations and spacing, individual circles or ovals atdiffering orientations and spacing, overlaid geometries, gradientfeatures, and a linear gradient feature having a Gaussian-typetransmission profile.

In the example of the planarization plate 300 in FIG. 3 and FIGS. 4A-4C,the bonded surface 390 may be the preferred surface for the location ofthe geometry. The planarization plate's main function is to controlwavefront distortion. The plate's bonded surface 390 may be a preferredlocation for multiple reasons. The planarizer glass may be far lesscostly and much lower risk to use than the assembled PBS. Theplanarization plate may be small, thin, and lightweight which makes iteasy to handle. It may be simpler to add features on a small, thin,lightweight piece of planarization plate glass, compared to that of thelarge, heavy, bulky, and costly assembled PBS. One example of how thegeometry may be added is by utilizing metal on glass lithography where ametal geometry is patterned and etched. It may be important that thegeometry be as close in proximity to the PBS seam as possible tominimize the separation between the feature and seam to achieve maskingand light scatter control at the same special location. This is anotherreason why the bonded surface, which is closest to the seam, may bepreferred. However, in some embodiments, the geometric feature may belocated not in close proximity to the seam.

The disclosed bonding method may incorporate a modified planarizationplate. For example, the bonded surface side of the glass plate may beprocessed using photolithography methods. The lithography processingcould utilize any single or combined geometry examples defined above, oradditional geometries determined to be effective may be used. Theplate's non-processed surface can receive a thin film anti-reflectivecoating to minimize input reflections as the light enters the PBS. Forexample, upon completion of the photolithography process, the plate mayyield a chrome or chrome oxide feature at the center of the plate's longaxis. This feature, which is designed to uniformly scatter or diffuselight along its edge, may then be aligned to the PBS seam during thebonding process to attach the planarization plate to the PBS.

FIG. 8 illustrates a plot 800 of schematic examples of geometries in adiffracting element that may suppress PBS seam diffraction. Care isneeded when establishing the feature height (h) with relation to theprojector lens pupil size and projection distance to the screen. Thereare at least two purposes of the feature. The first purpose is to maskthe seam region of the PBS either fully or partially, and the secondpurpose is to intentionally scatter light about the seam region in auniform and well-controlled manner. However, if the feature becomes toolarge in relation to the pupil size, the amount of light that is beingblocked may become visible on-screen as a dark band. It is well known inthe relevant art that straight lines diffract light in a unidirectionalfashion, and sharp points directionally concentrate scatter. For thesereasons, linear edges and sharp angles may be avoided. In FIG. 8 ,Example 2 shows geometries with linear edges and sharp angles, andExample 3 shows geometries with soft edges and rounded points. Example 6and Example 7 demonstrate a similar relationship as to Example 2 andExample 3. Example 5 illustrates a gradient geometry where thecenter-most region of the geometry yields the smallest amount of lighttransmission, and the top and bottom boundaries of the center regionbegin the locations where light transmission gradually increases.Example 5 could also be described as a neutral density gradient wherethe central region blocks a high percentage of light and, moving awayfrom the central region, the gradient transitions from a high percentageto a low percentage of light blockage.

FIG. 9 demonstrates example diffraction behavior caused by a linearfeature 910 and a sinusoidal feature 920. The scatter off of the linearfeature 910 shown in Example 1 is unidirectional and results in apparent“Image Flare” as well as diffractive banding on-screen. Example 2demonstrates that the scatter off the curved sinusoidal feature 920 israndomized and results in diffuse uniform light behavior. Light scatterin Example 2 is less apparent to the viewer and diffractive banding maybe suppressed on-screen.

FIG. 10 illustrates example on-screen light behavior resulting from ahorizontal linear feature 1010, a vertical linear feature 1020, asinusoidal feature 1030 and a linear edge sharp point feature 1040.These illustrations depict the difference between unidirectionaldiffraction and uniform scatter by diffuse or randomized diffraction.Additionally, a beneficial light attenuation may exist due to theobstructing feature also reducing the brightness of the on-screenscatter.

FIG. 11 schematically demonstrates a sinusoidal mask 1100. In thisillustration the sinusoidal geometry is patterned onto the center of theplanarization plate. During the PBS assembly process the sinusoidalgeometry on the planarization plate may be microscopically aligned tothe seam at the PBS entrance. In this example, the sinusoidal geometrycompletely covers the seam and light diffraction becomes randomized andscattered in a controlled fashion from the sine wave edges of thegeometry.

FIG. 12 illustrates an example 1200 of the on-screen “Image Flare”suppression obtained by utilizing the disclosed PBS bonding process. Thegeometry design randomizes scatter to create a uniform spraying of lightand overlap about the projected image which may be far less visible by aviewer. Additionally, a beneficial light attenuation may exist due tothe obstructing feature also reducing the brightness of the on-screenscatter.

FIG. 13 demonstrates an example 1300 of the effect of the PBS seam. Theresulting diffraction pattern width as observed on-screen can bedescribed as the size of the diffraction/scatter on-screen caused by thelinear obstructing feature as it relates to distance from the source,and can be expressed by the following: D=FASradians×T, where FAS is FullAngle Spread and T is distance 1320 from pupil (p) 1310 to screen 1330.Here, FAS is derived from the feature obstruction size (σ) in relationto the wavelength of light (λ). For a practical example, if σ is equalto 0.050 mm, p is equal to 25 mm, λ is equal to 0.0005 mm, and T isequal to 12,200 mm; therefore, D equals 122 mm.

FIG. 14 is a plot 1400 of an example of a pattern of geometric featuresthat can be used as an approach to suppress the unidirectionaldiffraction caused by a PBS seam. Consideration should be made for thefeature layout for the design to be effective at yielding theappropriate amount of controlled scatter to suppress the observedscatter generated at the PBS seam. If the geometric sizes become largethere may be a threshold where the region of blocked light becomesvisible on-screen and may manifest itself as a dark band. Ideally, thegeometries should be large enough to cover or mask the seam with a ratioof overlap that is proportional to the PBS seam width. FIG. 14 shows twoexamples of these relationships. Example 1 can be referred to as an openpattern, where the geometries that align to the seam do not completelycover the seam and contain a spatial pitch. Example 2 may be referred toas a closed pattern where there exists a solid pattern where a linegeometry completely covers the seam and the line boundaries are designedto uniformly control and scatter light. With respect to geometry sizesor line boundary designs, the diffraction caused by these elements mayincrease as the size and/or pitch decrease. A first-order pass of thegeometric relationship can be expressed though the following definitionsin Example 1 and Example 2: where, h˜p/100, s˜0.001 T/Dmm, and d iss/4<d<s/2, and d′ is ˜≤2d. Here, h is total geometry width, s is thepitch between geometries, d is width of the central-most geometry to bealigned to the seam and d′ are the size of the features or boundariesthat uniformly scatter light to suppress diffraction caused by the seam.For example, using D, T, and p values from the description of FIG. 13above, then h is ˜0.250 mm, s is 0.100 mm, d is 0.050 mm and d′ is0.025-0.050 mm. In FIG. 14 , Examples 2, 3 and 4 demonstrate differentconfigurations of spatial size/amplitude (d, d′) and pitch/frequency (s)for a given feature. These feature relationships may directly impact theeffectiveness of the geometry solution for unidirectional scatter.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

The invention claimed is:
 1. A stereoscopic image apparatus forprojecting a stereoscopic image towards an image-forming surface, thestereoscopic image apparatus comprising: a polarization beam splitter(PBS) adapted to split an incident image light into a transmitted lighthaving a first state of polarization, and first and second reflectedlight having a second state of polarization, the second state beingdifferent from the first state, wherein the polarization beam splittercomprises at least three prisms that define first and secondpolarization beam splitting surfaces arranged at an angle to each other,and a junction of the first and second polarization beam splittingsurfaces is located on a path of the incident image light and forms aseam; a diffracting element located in the input light path and prior tothe seam and aligned with the seam, wherein the diffracting elementcomprises a geometry that performs at least one of blocking a portion ofthe image light and scattering a portion of the image light; a firstreflective member in the first exit light path; and a second reflectivemember in the second exit light path, wherein light reflected by thefirst and second reflective members form a first stereoscopic image onthe image-forming surface.
 2. The stereoscopic image apparatus of claim1, wherein the geometry comprises at least one of a sawtooth geometry,an offset sawtooth geometry, a sine wave geometry, an offset sine wavegeometry, a square wave geometry, an offset square wave geometry, astepped square wave geometry, individual lines at differing orientationsand spacing, individual circles or ovals at differing orientations andspacing, overlaid geometries, gradient features, and a linear gradientfeature having a Gaussian type transmission profile.
 3. The stereoscopicimage apparatus of claim 1, wherein the at least three prisms comprise:a top prism having a first surface adapted to receive the image light onthe input light path and a second surface adapted to direct the imagelight along the first exit light path, wherein the first surface of thetop prism and the second surface of the top prism meet to form a firstedge, a bottom prism having a first surface adapted to receive the imagelight on the input light path and a second surface adapted to direct theimage light along the second exit light path, wherein the first surfaceof the bottom prism and the second surface of the bottom prism meet toform a second edge, and wherein the first edge and the second edge meetto form the seam, and a center prism between the top prism and thebottom prism, the center prism having a first surface adjacent to thesecond surface of the top prism, and the center prism having a secondsurface adjacent to the second surface of the bottom prism, wherein thefirst surface of the center prism and the second surface of the centerprism meet to form a third edge, and wherein the third edge meets thefirst edge and the second edge at the seam.
 4. The stereoscopic imageapparatus of claim 3, wherein the PBS further comprises: a planarizationplate prior to the seam in the input light path, the planarization plateadjacent to the first surface of the top prism and further adjacent tothe first surface of the bottom prism.
 5. The stereoscopic imageapparatus of claim 4, wherein the diffracting element is located on asurface of the planarization plate.
 6. The stereoscopic image apparatusof claim 4, wherein the surface of the planarization plate is bonded tothe first surface of the top prism and further bonded to the firstsurface of the bottom prism.
 7. The stereoscopic image apparatus ofclaim 4, wherein the planarization plate comprises optical grade glass.8. The stereoscopic image apparatus of claim 4, wherein at least aportion of the surface of the planarization plate has an anti-reflectivecoating.
 9. The stereoscopic image apparatus of claim 3, wherein atleast a portion of the second surface of the top prism and at least aportion of the second surface of the bottom prism have a reflectivecoating.
 10. The stereoscopic image apparatus of claim 9, wherein thereflective coating comprises a multi-layer thin film reflective coating.11. The stereoscopic image apparatus of claim 9, wherein the reflectivecoating comprises a wire grid thin film metal.
 12. The stereoscopicimage apparatus of claim 3, wherein the first surface of the centerprism and the second surface of the center prism meet to form a thirdedge, and wherein the third edge meets the first edge and the secondedge at the seam.
 13. The stereoscopic image apparatus of claim 3,wherein the first surface of the center prism is optically bonded to thesecond surface of the top prism and wherein the second surface of thecenter prism is optically bonded to the second surface of the bottomprism.
 14. The stereoscopic image apparatus of claim 13, wherein anindex-matched optically clear adhesive is used for the optical bonding.15. The stereoscopic image apparatus of claim 1, wherein the diffractingelement comprises a chrome or chrome oxide feature produced by aphotolithography process.
 16. The stereoscopic image apparatus of claim1, wherein a stereoscopic image is formed by overlapping of the firststereoscopic image and a second stereoscopic image formed from thetransmitted light on the image-forming surface.
 17. The stereoscopicimage apparatus of claim 1, further comprising a polarization modulatoradapted to switch the polarization state of the image light on at leastone of the first exit light path and the second exit light path betweena first state of polarization and a second state of polarization. 18.The stereoscopic image apparatus of claim 17, wherein the polarizationmodulator comprises first, second and third polarization modulatorsconfigured to selectively switch the polarization states of thetransmitted light and the first and the second reflected light betweenthe first and the second states of polarization.
 19. The stereoscopicimage apparatus of claim 18, wherein the geometry comprises at least oneof a sawtooth geometry, an offset sawtooth geometry, a sine wavegeometry, an offset sine wave geometry, a square wave geometry, anoffset square wave geometry, a stepped square wave geometry, individuallines at differing orientations and spacing, individual circles or ovalsat differing orientations and spacing, overlaid geometries, gradientfeatures, and a linear gradient feature having a Gaussian typetransmission profile.
 20. A stereoscopic image apparatus comprising: apolarizing beam splitter (PBS) adapted to split an incident image lightinto a transmitted light having a first state of polarization, and firstand second reflected light having a second state of polarization, thesecond state being different from the first state, wherein thepolarization beam splitter comprises: a top prism having a first surfaceadapted to receive the image light on the input light path and a secondsurface adapted to direct the image light along the first exit lightpath, wherein the first surface of the top prism and the second surfaceof the top prism meet to form a first edge, a bottom prism having afirst surface adapted to receive the image light on the input light pathand a second surface adapted to direct the image light along the secondexit light path, wherein the first surface of the bottom prism and thesecond surface of the bottom prism meet to form a second edge, andwherein the first edge and the second edge meet to form a seam, and adiffracting element prior to the seam, located in the input light path,the diffracting element aligned with the seam, wherein the diffractingelement comprises a geometry that performs at least one of blocking aportion of the image light and scattering a portion of the image light;a first reflective member in the first exit light path; and a secondreflective member in the second exit light path, wherein light reflectedby the first and second reflective members form a single stereoscopicimage on an image-forming surface.